The invention relates to a device for joining, by friction stir welding, at least two workpieces, which are disposed adjacent one another in a contact area, including a rotatable shaft having a driven end and, at the opposite end, a pin-like projection which is in contact with the workpieces in the contact area, and, upon rotation of the shaft, heats and plasticizes the workpiece materials in the contact area.
EP-B-0615 480 discloses a friction stir welding procedure (FSW) which has been known for several years. Originally, for friction welding two workpieces which were to be joined were moved relative to each other in the area in which they were to be joined while they were pressed against each other with a predetermined force. As a result of the heat generated by the friction, the materials of the workpieces were finally plasticized in the joining area. Upon sufficient plastification of the materials of the workpieces in the joining area, the workpiece materials are sufficiently mixed in the jointure area so that, upon cooling, the desired weld connection between the two workpieces is established.
In a friction stir welding device, a pin-like projection is driven by a drive, or respectively, motor at a suitably high speed so that two adjacent workpieces between which the pin-like projection is moved for example by a special guide structure or by a robot are joined in the butt area. When the workpiece materials in the butt are are sufficiently heated by the friction generated by the rotating pin-like projection so that the yare plasticized, the pin-like projection is moved along the butt edges of the two work pieces so as to form a longitudinal weld seam.
It is known to interconnect two or more metallic workpieces by spot-weld connections which, in comparison with rivet or bolt connections, have the advantage that no additional joining elements (rivet or, respectively, bolts or screws) are needed. In addition, the spot welding process is substantially faster since no bores have to be drilled, no rivets or screws have to be inserted into the bores and no rivet heads have to be formed and bolts or screws have to be mounted in order to engage the two workpieces with each other.
While two workpieces of iron or steel or alloys thereof which are disposed on top of each other can be easily joined by commercially available spot welding apparatus, two or more workpieces of aluminum alloys as they are used in the airplane or motor vehicle manufacture are difficult to join by the electric spot welding apparatus.
The reason is that aluminum and aluminum alloys form with the oxygen of the ambient air an oxide surface layer which, on one hand, forms a substantial electric resistance between the workpieces to be joined. On the other hand, the oxides are contained in the plasticized material of the workpieces so that aluminum oxide is disposed in the jointure between the two workpieces at relatively high local concentrations. As a result, the spot welding joints of workpieces of aluminum or aluminum alloys do not have the strength required for many applications.
For this reason, particularly in the airplane manufacture where the highest quality joints are required, the connection of aluminum workpieces by spot welding has been abandoned and the workpieces are again being joined by rivets. The same is true for the motor vehicle manufacture although aluminum is being increasingly used for vehicle chassis and body parts.
It is further known that, particularly in the manufacture of motor vehicles, increasingly manufacturing robots are used which perform various assembly operations including the joining of workpieces. The manufacturing robots include tool heads on which the respective special tool or equipment is mounted for performing particular tasks such as the joining of two or more workpieces. Such equipment has been employed so far in connection with robots only for establishing conventional spot welding connections, which are formed by conventional electric spot-welding apparatus for the jointure of workpieces of iron or steel, that is, essentially for joining iron or steel sheets. Workpieces of aluminum or aluminum alloys as they are used more and more also in the construction of motor vehicles are not being joined by spot welding because of the disadvantages mentioned earlier. It has furthermore been assumed that the friction stir welding procedure is not suitable for the formation of spot-like weld joints.
It is the object of the present invention therefore to provide a device for establishing, in a simple manner, high-strength spot weld connections between workpieces wherein the jointure between the workpieces is essentially free of oxides of the workpiece materials and which can be operated essentially continuously and in a simple manner also in connection with a manufacturing robot.
In a device for joining, by friction stir welding, at least two workpieces, a support structure is provided with a head portion which includes drive means and a shaft which is rotatable by the drive means has a pin-like projection projecting toward a support member of the support structure for supporting workpieces to be joined by friction stir welding and guide means are provided for moving the rotating pin-like projection into contact with, and into, the workpieces while plasticizing the workpieces in the contact area to form thereby a spot weld by friction stir welding.
With the device according to the invention, two or more workpieces can be joined by spot welds using the friction stir welding procedure. Such spot welds could be formed so far only by electric spot welding apparatus, which however, as described above, could be used for certain materials, particularly aluminum and aluminum alloys only in a very limited way because of the disadvantages mentioned earlier. With the solution according to the invention, it is possible to join workpieces by spot welding which, so far, could not be joined by the available spot welding methods, particularly workpieces of aluminum or aluminum alloys without the need for a particular preparation of the jointure area. There is no need to drill bores into the workpieces, no additional connecting means are required and no separate work procedure is needed such as the formation of a rivet head or of a screw connection including a nut. With the device, spot-like weld connections can be formed by friction stir welding or the jointure can be established by a seam along a certain welding length depending on the control for the movement of the guide means which moves the pin-like projections of the friction stir welding device.
In an advantageous embodiment of the device, the guide means is a pivot arm, which is pivotable about an axis that extends parallel to the axis of the pin-like projection. The pivot arm, which itself is rigid, supports and guides the pin-like projection such that it is guided along a circular path. However, it is pointed out that the pivot arm does not necessarily have to be pivotable about an axis which extends parallel to the axis of the pin-like projection. Rather, the pivot arm may also be movable linearly and also along the pivot axis so that a linearly extended spot-like welding joint can be established.
If the pivot arm performs a circular motion around an axis parallel to the axis of the pin-like projection, it is advantageous if the arc of the circle of the movement corresponds essentially to the length of the friction welding seam.
It is advantageous if the pivot arm at whose free end the pin-like projection is arranged, is pivotable independently of a robot, on which the device is mounted, so that the pin-like projection can be moved to some extent without the need for controlling and actuating the robot. To this end, the device includes expediently a piston-cylinder structure by way of which the pin-like projection is movable along the workpieces or relative to the workpieces. However in place of a pneumatic or hydraulic piston-cylinder operating structure, an electromechanically operated actuator may be provided which is driven by an electric motor.
In order to ensure that, during the quasi-spot welding step, the workpieces to be joined are pressed against one another with high precision so that they can move neither relative to each other nor relative to the welding tool formed by the pin-like projection, it is advantageous if the device includes a clamping lever by means of which the workpieces can be biased toward each other and against a support member.
The biasing of the workpieces against each other and into engagement with a support member is preferably also achieved by means of an actuator, which is part of the device. Preferably it is also a piston-cylinder structure by which the clamping lever is movable between a clamping and a release position. However, also in this case, the actuator may be an electromechanical operating structure.
In another advantageous embodiment of the invention, the device includes a head portion which includes the drive for the pin-like projection. The head portion is preferably so supported that it can move the pin-like projection also axially for forming the spot welding joint and for retracting the pin-like projection after completion of the spot welding joint. In this way, also the movement of the pin-like projection toward the workpieces to be joined, and away therefrom after completion of the spot-welding joint, is initiated by the device itself so that no control and no movement of the robot is needed for that purpose. Also, the rotation of the pin-like projection is preferably provided for by a drive disposed in the head portion.
In order to ensure under all operating conditions and for all workpiece thicknesses, an orthogonal or possibly about 3xc2x0 angled engagement of the pin-like projection with the surface of the workpiece at least in the area where the pin-like projection comes into contact with the surface of the closest workpiece, the head portion is preferably movable by means of spaced yoke-like pivot elements, which form, together with the head portion and a rotating member which is rotatable about the pivot axis of the pivot arm, an essentially parallelogram-like structure. In this way, the selected angle of the axis of the pin-like projection relative to the workpieces to be joined remains unchanged.
Finally it is also advantageous if the head portion is movable by a device-internal actuator, which may also be pneumatically or hydraulically operated by a piston-cylinder structure or by a motor operated electromechanical actuator.
With the use of the device according to the invention in connection with a manufacturing robot, the manufacturing robot only needs to move the device to the location and position the device properly at such location, where the spot or spot-like welding connection of the workpiece is to be established.
Below, the invention will be described in greater detail on the basis of the accompanying schematic drawings.